The present invention relates generally to electronic ballasts for powering fluorescent lamps. More particularly, the present invention relates to ballast circuitry for detecting asymmetric end-of-life conditions in a fluorescent lamp and disabling the ballast accordingly.
In the field of electronic ballasts, an asymmetric end-of-life (AEOL) condition is associated with the operating behavior of a fluorescent lamp as it approaches the end of its life cycle. In T5 fluorescent lamps, for example, a very important end-of-life lamp characteristic is electrode degradation. With electrode degradation, the electrode operates like a large rectifying resistor. Thus, in current-controlled ballasts, the ballast will supply more power to maintain a nominal current. This extra power is dissipated in the degraded electrode. With enough power dissipation, the electrode will become hot and cause the lamp glass or lamp holder to melt or otherwise become physically unstable. This causes a dangerous situation, and accordingly AEOL protection circuitry has been developed and is known in the art to detect an asymmetric voltage disparity across lamps in contrast to the sinusoidal voltage which normally appears, and to proactively shut down the electronic ballast.
It is important to design AEOL protection circuitry to be robust and various such circuits as are conventionally known in the art are consequently more complex or less cost effective than is otherwise desirable.